Despite being crucial in an optical lithography process, “dose” has remained a relative concept in the computational lithography regime. It usually takes the form of a percentage deviation from a pre-identified “nominal condition” under the same illumination shape. Dose comparison between different illumination shapes has never been rigorously defined and modeled in numerical simulation to date. On the other hand, the exposure-limited nature of EUV lithography throughput demands the * illumination shape being optimized with the physical dose impact consciously taken into consideration. When the projection pupil is significantly obscured (as in the ASML EXE high NA scanner series), the lack of a proper physical dose constraint may lead to suboptimal energy utilization during exposure. In this paper, we demonstrate a method to accurately model the physical dose in an optical lithography process. The resultant dose concept remains meaningful in the context of a changing illumination pupil, which enables co-optimization of imaging quality and a throughput metric during the Source-Mask Optimization (SMO) phase, known as the Dose-Aware SMO. With a few realistic test cases we demonstrate the capability of Dose-Aware SMO in terms of improving EUV throughput via reducing the effective exposure time, in both regular and obscured projection systems. The physical dose modeling capability in computational lithography not only addresses those immediate challenges emergent from EUV throughput, but also opens the gate towards a broad class of exciting topics that are built upon physical dose, such as optical stochastic phenomena and so on.
Over the years, lithography engineers have continued to focus on CD control, overlay and process capability to meet node requirements for yield and device performance. Previous work by Fukuda1 developed a multi-exposure technique at multi-focus positions to image contact holes with adequate DOF. Lalovic2 demonstrated a fixed 2-wavelength technique to improve DOF called RELAX. The concept of multi-focal imaging (MFI) was introduced3 demonstrating two focal positions are created that are averaged over the exposure field, this wavelength “dithering” approach which can be turned on and off, thus eliminating any potential scanner calibration issues.
In this work, the application of this imaging method (1 exposure-2 focus positions) can be used in thick photoresist and high aspect ratio applications. An example of thick photoresist imaging is shown in figure 1. We demonstrate 5um line and space features in 10um of photoresist at 3 different imaging conditions. On the left, single focus imaging (SFI) at best dose and focus, the center image which is also SFI but at a defocus of +3.2um. On the right is MFI with 2 focus positions of 0 and 2.8um. Here we can see a significant improvement in the SWA linearity and image profile quality. A second example of high aspect ratio imaging using MFI is shown in figure 2. The aspect ratio of 13:1 is shown for this. The use of Tachyon KrF MFI source – mask optimization flow will be reviewed to demonstrate optimum conditions to achieve Customer required imaging to meet specific layer requirements.
Extreme ultraviolet lithography (EUVL) uses a 13.5nm exposure wavelength, all-reflective projection optics, and a reflective mask under an oblique illumination with a chief ray angle of about 6 degrees to print device patterns. This imaging configuration leads to many challenges related to 3D mask topography. In order to accurately predict and correct these problems, it is important to use a 3D mask model in full-chip EUVL applications such as optical proximity correction (OPC) and verifications. In this work, a fast approximate 3D mask model developed previously for full-chip deep ultraviolet (DUV) applications is extended and greatly enhanced for EUV applications and its accuracy is evaluated against a rigorous 3D mask model.
Implant layer patterning is becoming challenging with node shrink due to decreasing critical dimension (CD) and usage
of non-uniform reflective substrates without bottom anti-reflection coating (BARC).
Conventional OPC models are calibrated on a uniform silicon substrate and the model does not consider any wafer
topography proximity effects from sub-layers. So the existing planar OPC model cannot predict the sub-layer effects
such as reflection and scattering of light from substrate and non-uniform interfaces. This is insufficient for layers without
BARC, e.g., implant layer, as technology node shrinks.
For 45-nm and larger nodes, the wafer topography proximity effects in implant layer have been ignored or compensated
using rule based OPC. When the node reached 40 nm and below, the sub-layer effects cause undesired CD variation and
resist profile change. Hence, it is necessary to model the wafer topography proximity effects accurately and compensate
them by model based OPC. Rigorous models can calculate the wafer topography proximity effects quite accurately if
well calibrated. However, the run time for model calibration and OPC compensation are long by rigorous models and
they are not suitable for full chip applications. In this paper, we demonstrate an accurate and rapid method that considers
wafer topography proximity effects using a kernel based model. We also demonstrate application of this model for full
chip OPC on implant layers.
We describe novel methods for waveform synthesis and detection relying on longitudinal spectral decomposition of subpicosecond optical pulses. Optical processing is performed in both all-fiber and mixed fiber/free-space systems. Demonstrated applications include ultrafast optical waveform synthesis, microwave spectrum analysis, and high-speed electrical arbitrary waveform generation. The techniques have the potential for time bandwidth products ≥104 due to exclusive reliance on time-domain processing. We introduce the principles of operation and subsequently support these with results from our experimental systems. Both theory and experiments suggest third order dispersion as the principle limitation to large time-bandwidth products. Chirped fiber Bragg gratings offer a route to increasing the number of resolvable spots for use in high speed signal processing applications.
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